DESCRIPTION

The SETTRANSACTION command sets the characteristics of the current
transaction. It has no effect on any subsequent transactions. SETSESSIONCHARACTERISTICS sets the default transaction characteristics
for subsequent transactions of a session. These defaults can be
overridden by SETTRANSACTION for an individual transaction.
The available transaction characteristics are the transaction isolation
level and the transaction access mode (read/write or read-only).
The isolation level of a transaction determines what data the
transaction can see when other transactions are running concurrently:
READCOMMITTED
A statement can only see rows committed before it began. This is
the default.
SERIALIZABLE
All statements of the current transaction can only see rows
committed before the first query or data-modification statement
was executed in this transaction.
The SQL standard defines two additional levels, READ UNCOMMITTED and
REPEATABLE READ. In PostgreSQL READ UNCOMMITTED is treated as READ
COMMITTED, while REPEATABLE READ is treated as SERIALIZABLE.
The transaction isolation level cannot be changed after the first query
or data-modification statement (SELECT, INSERT, DELETE, UPDATE, FETCH,
or COPY) of a transaction has been executed. See in the documentation
for more information about transaction isolation and concurrency
control.
The transaction access mode determines whether the transaction is
read/write or read-only. Read/write is the default. When a transaction
is read-only, the following SQL commands are disallowed: INSERT,
UPDATE, DELETE, and COPY FROM if the table they would write to is not a
temporary table; all CREATE, ALTER, and DROP commands; COMMENT, GRANT,
REVOKE, TRUNCATE; and EXPLAIN ANALYZE and EXECUTE if the command they
would execute is among those listed. This is a high-level notion of
read-only that does not prevent all writes to disk.

NOTES

If SETTRANSACTION is executed without a prior STARTTRANSACTION or
BEGIN, it will appear to have no effect, since the transaction will
immediately end.
It is possible to dispense with SETTRANSACTION by instead specifying
the desired transaction_modes in BEGIN or STARTTRANSACTION.
The session default transaction modes can also be set by setting the
configuration parameters default_transaction_isolation and
default_transaction_read_only. (In fact SETSESSIONCHARACTERISTICS is
just a verbose equivalent for setting these variables with SET.) This
means the defaults can be set in the configuration file, via ALTERDATABASE, etc. Consult in the documentation for more information.

COMPATIBILITY

Both commands are defined in the SQL standard. SERIALIZABLE is the
default transaction isolation level in the standard. In PostgreSQL the
default is ordinarily READ COMMITTED, but you can change it as
mentioned above. Because of lack of predicate locking, the SERIALIZABLE
level is not truly serializable. See in the documentation for details.
In the SQL standard, there is one other transaction characteristic that
can be set with these commands: the size of the diagnostics area. This
concept is specific to embedded SQL, and therefore is not implemented
in the PostgreSQL server.
The SQL standard requires commas between successive transaction_modes,
but for historical reasons PostgreSQL allows the commas to be omitted.